The neonatal proximal tubule has a lower rate of bicarbonate absorption and Na+/H+ antiporter activity than the proximal tubule of adult animals. Two isoforms of the Na+/H+ antiporter have been localized to the proximal tubule. NHE3 is located on the apical membrane, whereas NHE1, the isoform found on most mammalian cells, is present on the basolateral membrane. The Na+/H+ antiporter isoforms that increase with renal maturation are unknown. The purpose of the present study was to examine the maturation of rabbit renal cortical NHE3 and NHE1 mRNA and protein abundance and to determine whether the rate of maturation of these isoforms was affected by glucocorticoids. Renal cortex from neonatal rabbits (1 wk) had approximately one-fourth the NHE3 mRNA and protein abundance as that from adult animals. Renal cortical NHE1 mRNA and protein abundance did not change significantly during maturation. Glucocorticoids have been shown to accelerate the maturation of neonatal bicarbonate absorption and apical membrane Na+/H+ antiporter activity. Daily subcutaneous administration of dexamethasone starting at 4 days of age (10 micrograms/100 g body wt) for 3 days and 2 h before being killed resulted in a twofold increase in NHE3 mRNA abundance and a threefold increase in NHE3 protein abundance. NHE1 mRNA and protein abundance were unaffected. These data show that there is selective maturation of NHE3 during renal cortical development, which can be accelerated by administration of glucocorticoids.